The overall objectives of these studies are to identify and characterize the lymphoid cells which participate in the immune response of the rabbit. The proposed work for the second year of this investigation will be concerned with the following areas: 1) Rabbit cells analogous to mouse T and B cells will be isolated using goat antirabbit thymocyte serum to remove "T" cells and nylon wool columns which selectively remove "B" cells. The functions of these isolated cell populations in the in vitro plaque-forming cell (PFC) response and proliferative responses to mitrogens, specific antigens and in mixed lymphocyte reactions will be defined. 2) The factors which determine the immunoglobulin class and affinity of antibody, e.g., antigen dose, relative number of "T" cells, etc., will be examined. 3) "T" cells specific for antigens will be isolated by affinity chromatography and examined in the in vitro PFC system and for antigen specific proliferation. 4) The relationships between mixed lymphocyte reactions and graft rejection will be determined. Both outbred and inbred (strain III, Jackson Laboratory) will be included. 5) Rabbit B2-microglobulin will be isolated and antisera prepared as the first phase in attempting to isolate rabbit histocompatibility antigens and to determine the relationship of B2-microglobulin to immune reactions of lymphoid cells.